I took a photo of the top of my larger Mercer and another of the center pocket. The center pocket is zippered so it's not completely wide open. Also, the center pocket has one smaller zippered pocked within it as well as a slip pocket and key fob. I find the slip pocket works well for my phone, as it's more horizontal than the usual tiny phone pockets. You are right, though, in that there is no way to close the main part of the bag. The two outside "pockets" are open, but the bag is structured enough that it doesn't feel like things will be falling out everywhere. The only gripe I have about this bag is that the center pocket isn't attached to the bottom of the bag so if you put smaller things (coupons, receipts, etc) in either of the outer pockets, they can find their way down under the center pocket and it's kind of a PIA to reach. Still, I love the bag and find it has enough organization for my things. If you're familiar with the Hamilton, I find the Mercer to be less open at the top than that even though the Hamilton has the magnetic closure.
View attachment 3705827 View attachment 3705828
One thing to note with the smaller Mercer is that it has card slots in the front pocket, but the center pocket doesn't have the same zip and slip pockets like her larger sister.